1. Field of the Invention
The present invention relates to a DC-DC converter which performs PWM (Pulse Width Modulation) control on a main switch element, particularly to a DC-DC converter which reduces switching loss.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2001-169545 discloses a DC-DC converter used as a DC stabilized power supply circuit. FIG. 1 is a circuit diagram of the DC-DC converter described in Japanese Unexamined Patent Application Publication No. 2001-169545. A primary coil 3 of a transformer 2 is connected to a main switch element 5. A secondary coil 7 of the transformer 2 is connected to a rectifying and smoothing circuit 40 including a rectification-side synchronous rectifier 8, an inverter 9, a commutation-side synchronous rectifier 10, a smoothing capacitor 12, and a choke coil 13. The rectifying and smoothing circuit 40 is connected to a load 15.
A tertiary coil 18 of the transformer 2 is connected to an output voltage detection circuit 41 which detects an output voltage Vout by using diodes 21 and 19, a capacitor 20, a choke coil 22, and resistors 23 and 24.
A divided voltage output from the resistors 23 and 24 is connected to a control circuit 42 including an operational amplifier 25, a reference power supply 26, a comparator 27, a resistor 28, a capacitor 29, and a triangular wave oscillator 30.
In the DC-DC converter, the switching frequency is set by the triangular wave oscillator 30, and the PWM control is performed on the basis of feedback signals. A resonance voltage is generated in the OFF period of the main switch element 5, and resonant reset is performed in the OFF period. The switching frequency is fixed. If a primary-side input voltage varies, the ON duty ratio is changed by the PWM control to maintain a constant output voltage. When the turn ratio of the primary and secondary coils is represented by N, a relationship Vout/Vin=N×Ton/T is established among the input voltage Vin, the output voltage Vout, the period T, and the ON time Ton. If the switching frequency is fixed, therefore, the period T is fixed, and the output voltage Vout is controlled by the ON time Ton. Herein, the ON duty ratio is defined as Ton/T.
FIGS. 2A and 2B are waveform charts of a gate voltage Vg and a drain-source voltage Vds of the main switch element 5 illustrated in FIG. 1. With reference to FIGS. 2A and 2B, description will be made of issues and problems with a PWM-controlled forward converter as described in Japanese Unexamined Patent Application Publication No. 2001-169545.
First, if the input voltage is low, the ON duty ratio of the PWM is increased, as illustrated in FIG. 2A. That is, the ON time of the main switch element 5 is increased, and the OFF time thereof is reduced. With the reduction of the OFF time, the main switch element 5 is turned on before the completion of resonant reset (reset of the excitation of a transformer caused by the flow of resonance current after the turn-off of a main switch element). Therefore, switching is performed with a high value of the drain-source voltage Vds of the main switch element 5 (voltage Vs1 in FIG. 2A), and a switching loss occurs.
At the time of resonant reset, a voltage combining the input voltage and the resonance voltage (voltage Vp1 in FIG. 2B) is applied between the drain and source of the main switch element 5. If the input voltage is high, therefore, the peak value of the voltage generated by the resonant reset is increased. Accordingly, a high voltage resistant switch element is required. In general, a high voltage resistant switch element has a high Rdson (ON resistance). Therefore, the switching loss is increased.
As described above, the related art DC-DC converter which performs PWM control on a main switch element has a problem in that the switching loss occurs at both a high input voltage and a low input voltage.